The curing of coating systems which carry activated double bonds by actinic radiation, such as UV light, IR radiation or else electron beams, is known and is established in industry. It is one of the most rapid curing methods in coating technology. Coating compositions based on this principle are thus referred to as radiation- or actinically curing or curable systems.
Because of the environmental and economic requirements imposed on modern coating systems, that they should use as little organic solvents as possible, or none at all, for adjusting the viscosity, there is a desire to use coatings raw materials which are already of low viscosity. Known for this purpose for a long time have been polyisocyanates with an allophanate structure as are described, inter alia, in EP-A 0 682 012.
In industry these substances are prepared by reacting a monohydric or polyhydric alcohol with large amounts of excess aliphatic and/or cycloaliphatic diisocyanate (cf. GB-A 994 890, EP-A 0 000 194 or EP-A 0 712 840). This is followed by removal of unreacted diisocyanate by means of distillation under reduced pressure. According to DE-A 198 60 041 this procedure can also be carried out with OH-functional compounds having activated double bonds, such as hydroxyalkyl acrylates, although difficulties occur in relation to the preparation of particularly low-monomer-content products. Since the distillation step has to take place at temperatures up to 135° C., in order to be able to lower the residual isocyanate content sufficiently (<0.5% by weight of residual monomer), it is possible for double bonds to react, with polymerization, under thermal initiation, even during the purification process, meaning that ideal products are no longer obtained.
The preparation of low-monomer-content, allophanate-containing, polyurethane-based, radiation-curing binders is described in EP-A 0 867 457 and U.S. Pat. No. 5,739,251. These binders, however, do not carry activated double bonds but instead carry unreactive allyl ether groups (structure R—O—CH2—CH═CH2). It is therefore necessary to add reactive diluents (low molecular weight esters of acrylic acid), which introduce the required UV reactivity.
There has also been no paucity of attempts to prepare allophanates indirectly, from other isocyanate derivatives, urethanes and isocyanates. For instance, EP-A 0 825 211 describes a process for synthesizing allophanate structures from oxadiazinetriones, although no mention is made there of radiation-curing derivatives containing activated double bonds. Transposition to the particular circumstances of radiation-curing systems is described in German application No.: 10246512.6, unpublished at the priority date of the present specification.
Another route is the opening of uretdiones (cf. Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium 2001, 28th, 405-419, and also US-A 2003 0153 713) to give allophanate structures, which have also been already successfully transposed to radiation-curing systems (German application No.: 102004012903, unpublished at the priority date of the present specification).
Both routes require high-grade raw materials as starting material and lead only to an allophanate product which is rich in by-products.
U.S. Pat. No. 5,777,024 describes the preparation of low-viscosity radiation-curing allophanates by reacting hydroxy-functional monomers which carry activated double bonds with isocyanate groups of allophanate-modified isocyanurate polyisocyanates. The radicals attached via the allophanate groups are saturated, and so any possible higher functionality is foregone.
EP-B 694 531 describes a multi-stage process for preparing hydrophilicized allophanates containing radiation-curing groups. In that case, however, first an NCO— and acrylate-functional urethane is prepared, which is then hydrophilicized and subsequently allophanatized following addition of a further NCO— and acrylate-functional urethane. As the process temperature for the allophanatization, temperatures of 100 to 110° C. are specified.
It was the object of the present invention, then, to provide, on the basis of readily available raw materials in one operation at a moderate temperature of below 100° C., and without a distillation step, an NCO-group-free, high-functionality allophanate mixture containing groups crosslinkable by actinic radiation (radiation-curing groups) as a radiation-curing binder, the intention being that this binder should have a residual diisocyanate monomer content of less than 0.5% by weight. The viscosity of this product ought to be sufficiently low, i.e. below 200 000 mPas @23° C., that it can be processed at room temperature even without addition of solvent.